Cadmium-Tolerant Rhizospheric Bacteria of the C3/CAM Intermediate Semi-Halophytic Common Ice Plant (Mesembryanthemum crystallinum L.) Grown in Contaminated Soils

Abstract

The common ice plant, Mesembryanthemum crystallinum L., has recently been found as a good candidate for phytoremediation of heavy-metal polluted soils. This semi-halophyte is a C3/CAM (Crassulacean acid metabolism) intermediate plant capable of tolerating extreme levels of cadmium in the soil. The aim of the work was to obtain and characterize novel, Cd-tolerant microbial strains that populate the root zone of M. crystallinum performing different types of photosynthetic metabolism and growing in Cd-contaminated substrates. The plants exhibiting either C3 or CAM photosynthesis were treated for 8 days with different CdCl2 doses to obtain final Cd concentrations ranging from 0.82 to 818 mg⋅kg–1 of soil d.w. The CAM phase was induced by highly saline conditions. After treatment, eighteen bacterial and three yeast strains were isolated from the rhizosphere and, after preliminary Cd-resistance in vitro test, five bacterial strains were selected and identified with a molecular proteomics technique. Two strains of the species Providencia rettgeri (W6 and W7) were obtained from the C3 phase and three (one Paenibacillus glucanolyticus S7 and two Rhodococcus erythropolis strains: S4 and S10) from the CAM performing plants. The isolates were further tested for Cd-resistance (treatment with either 1 mM or 10 mM CdCl2) and salinity tolerance (0.5 M NaCl) in model liquid cultures (incubation for 14 days). Providencia rettgeri W7 culture remained fully viable at 1 mM Cd, whereas Rh. erythropolis S4 and S10 together with P. glucanolyticus S7 were found to be resistant to 10 mM Cd in the presence of 0.5 M NaCl. It is suggested that the high tolerance of the common ice plant toward cadmium may result from the synergic action of the plant together with the Cd/salt-resistant strains occurring within rhizospheral microbiota. Moreover, the isolated bacteria appear as promising robust microorganisms for biotechnological applications in bio- and phytoremediation projects.

Does Cd possess the stimulatory effect on gas exchange and photochemistry in C3-CAM intermediate plants?

“…However, this was observed only in mycorrhizal plants suggesting that mycorrhizal may positively in uence photochemical activity (Aloui et al 2011). Also, the rhizospheric bacteria may increase plants tolerance towards Cd (Supel et al 2022). Cd was shown to reduce electron ow through PSII and enhance non-photochemical quenching (Larsson et al 1998).…”

A Culturomics-Based Bacterial Synthetic Community for Improving Resilience towards Arsenic and Heavy Metals in the Nutraceutical Plant Mesembryanthemum crystallinum

“…Interestingly, different isolates could be recovered from the rhizosphere of plants displaying C3 or CAM metabolisms (the latter was preinduced by salt treatment). Although two Providencia strains were found among the most tolerant to Cd in C3 plants, Gram-positive species such as Paenibacillus and Rhodococcus were the most tolerant species to Cd in CAM plants [ 16 ]. In our culturomics approach, both Gram-positive and Gram-negative species were also present and exhibited the highest levels of metal tolerance.…”

“…Moreover, the effects were more pronounced in the presence of metals, supporting the application of these SynComs in situations of metal pollution. Analogously, metal-resistant PGPR, including Providencia rettgeri, Paenibacillus glucanolyticus, and Rhodococcus erythropolis, appear as promising robust microorganisms for biotechnological applications in phytoremediation projects using M. crystallinum plants [ 16 ].…”

“…The accumulation of toxic metals in plant tissues is important not only to establish the possible application of this plant in phytoremediation [ 15 , 16 ], but also to establish whether its cultivation in polluted soils could be safe; particularly considering that it has pharmaceutical applications [ 13 , 14 ]. Our results demonstrate that the plant accumulated metals mainly in roots, with a strong limitation of translocation to the shoot.…”

“…In addition, M. crystallinum is known for its cadmium and chromate phytoremediation capacity [ 15 ]. In this regard, several metal-resistant endophytes have been isolated from this plant in both metabolism states, C3 and CAM, which display good phytoremediation potential [ 16 ]. These bacteria include Providencia rettgeri , isolated during the C3 phase, and Paenibacillus glucanolyticus and Rhodococcus erythropolis isolated during the CAM phase.…”